Recent significant progress in conductor technology has resulted in three major high-temperature superconducting (HTS) conductors available in the field of commercial applications: Bismuth Strontium Calcium Copper Oxides (BSCCO), which comes in two stoichiometries and form factors, Bi2Sr2CaCu208+x (Bi-2212) round wire conductor and Bi2Sr2Ca2Cu3O10+x (Bi-2223) tape conductor, and Rare Earth Barium Copper Oxides (ReBCO) tape conductor. These HTS conductors have been developed by research institutions and industry focusing on a high degree of applicability, and substantial efforts are currently being made to commercialize these HTS conductors. Two dominant application areas are driving HTS technology: the first is power generation and transport, an area for which ReBCO is highly suitable, and the second is high field magnet systems, an area for which Bi-2212 and Bi-2223 are preferred. Bi-2212 round wire shows great potential particularly for high-field magnets where high-field homogeneity and long-term field stability are required, as is the case in nuclear magnetic resonance (NMR) magnets, which have become essential in biological and medical science as tools to understand and decipher protein structures to enable the development of improved medical treatments. Accordingly, there is a strong motivation in the market for generating conductors capable of providing increasingly higher magnetic fields. However, currently used superconducting magnets made with low temperature superconductors (LTS), including Nb—Ti and Nb3Sn, cannot operate in or generate fields above 25 T. In contrast to LTS conductors, HTS conductors retain the superconducting state at fields far above 100 T at liquid helium temperatures.
HTS conductors are produced in limited piece lengths, which are shorter than the lengths typically required to build high field magnet systems, commonly requiring several kilometers of conductor material. To provide a longer superconductor for use in high field magnet systems requires creating one longer length conductor by connecting multiple shorter length HTS conductors through the establishment of multiple electrical joints between the shorter HTS conductors. In conventional soldered electrical joints, ohmic losses in the joints contribute to the total heat generation of the coil formed by the HTS conductors which, in the best case, increases consumption of the cryogenic coolant and, in the worst case, may lead to premature quenching of the magnet, resulting in the loss of the superconducting properties and the magnetic field of the HTS conductor. As such, conventional resistive joints present a performance limiting factor in high-field coil design. Additionally, a magnet using resistive joints has to be operated with a power supply that is constantly powering the magnet in order to keep the operating current of the magnet constant. Providing a constant operating current requires the use of an extremely stable and expensive power supply and also requires a very advanced electronic control loop. In contrast, a fully superconducting magnet, having superconducting joints between the HTS conductors, can be operated in “persistence”, meaning that once the magnet reaches its operating current and target field, the coil terminals can be shorted and the power supply can be taken out of the loop. The operating current then stays within the magnet and, due to the low losses in the superconducting joints, decays only very little over time, which means that the magnet remains stable at its target field for an extended period of time. This is of particular importance for NMR type magnet systems.
Additionally, over time, ohmic losses in the electrical joints alter the current distribution in the magnet, leading to changes in the magnetic field profile and field instability, ultimately requiring complicated field compensation measures. These compensation measures would not be necessary if the HTS coil were capable of operating in “persistence”, resulting in very low losses, which can be achieved using superconducting electrical joints.
While methods are currently known in the art for creating superconducting joints in Bi-2212 round wire or tape conductors and also in Bi-2223 tape conductors, the known methods are impractical or incompatible with the current production techniques for HTS conductors. Known methods for creating superconducting joints in Bi-2223 tape conductors require an additional heat treatment of the joint after the coil is wound. This additional heat treatment is lengthy and complex, rendering it highly impractical for commercial production. Additionally, known methods for creating superconducting joints in Bi-2212 round wire or tape conductors are incompatible with the high pressure environment inside the furnace that is required during the heat treatment of the Bi-2212 required to establish the transport current carrying superconducting phase.
Accordingly, what is needed in the art is a system and method for establishing superconducting electrical joints in a Bi-2212 conductor, which introduces negligible ohmic resistance against the electrical current in the superconductor.